1. Field of the Invention
The present invention relates generally to a semiconductor light emitting element and a method for manufacturing the same.
2. Related Background Art
Semiconductor light emitting elements, including light emitting diodes (LEDs) for emitting visible light, are widely used as light sources for display purposes because of their excellent features, namely, compactness, low power consumption, high reliability, and so on. Once they are enhanced in luminosity, their application as outdoor displays and light sources for communication will be broadened exponentially. There are AlGaAs, GaAlP and GaP are examples of materials of high-luminance LED already brought into practice, and those for emitting light of red, orange, yellow, green and other colors are actually being supplied at low costs.
Recently, InGaAlP having a band structure of a direct transition type from red to green has been remarked as a high-luminance LED material in this wavelength range. For its crystalline growth, metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are currently used because, with liquid phase epitaxy (LPE) heretofore used to grow GaAlAs, GaP and other conventional materials, it is difficult to control the composition because of large segregation of Al. In MOCVD and MBE, InGaAlP is formed on a GaAs substrate that is in lattice-matching with InGaAlP.
However, such a GaAs substrate is opaque to light from an InGaAlP active layer. Therefore, there is a development of a method of removing the opaque GaAs substrate and instead bonding a GaP substrate transparent to light from the InGaAlP active layer to obtain a relatively high-luminance LED.
FIG. 16 is a diagram showing a LED using a GaP substrate 200 mentioned above. A light emitting layer 201 made of InGaAlP compound semiconductors and having an InGaAIP active layer is bonded on the GaP substrate. The light emitting layer 201 emits light by current injection from the p-side electrode 202 and the n-side electrode 203. The GaP substrate 200 is transparent to the light from the light emitting layer 201. The transparent substrate 200 has side surfaces 200A, 200B that are slanted. Slanting angles of these side surfaces 200A, 200B can be changed as desired. In the LED of FIG. 16, however, the side surfaces are shown as slanting by 45° relative to the plane shown as the top surface in FIG. 16 for purposes of easier explanation about plane orientation. This LED of FIG. 16, using the transparent substrate 200 and slanting the first side surface 200A and the second side surface 200B of the transparent substrate 200, enables extraction of light from the light emitting layer 200 also from the side surfaces 200A, 200B, and is therefore enhanced in light extraction efficiency. Moreover, light from the light emitting layer 201 is extracted also from the third side surface (not shown) opposed to the first side surface 200A and from the fourth side surface (not shown) opposed to the second side surface 200B, and the light extraction efficiency is enhanced even more. The GaP substrate 200 is a just substrate having no gradient in plane orientation. As shown in FIG. 17, the first side surface 200 is (1-11) oriented, the second side surface 200B is (111) oriented, the third side surface is (11-1) oriented, and the fourth side surface 200D is (1-1-1) oriented.
To increase the light extraction efficiency of a semiconductor light emitting element still more, also used is a method of providing surfaces of elements with a plurality of projections and depressions that are high or deep by approximately the emission wavelength (sub-micron). This contributes to improving the light extraction efficiency by enlarging the surface area of the element and thereby improving the transmission probability of light, or making use of a change ineffective refractive index. Even in the element shown in FIG. 16, light extraction efficiency can be enhanced by providing, for example, the first side surface 200A and the opposed third side surface 200C of the substrate 200 with such depressions and protrusions. In case of a GaP substrate having slanted side surfaces, it is known to be relatively easy to form depressions and protrusions on the (1-11) oriented surface and the opposed (11-1) oriented surface by wet etching. Therefore, with the element shown in FIG. 16, it is relatively easy to form such depressions and protrusions on the first side surface 200A and the opposed third side surface 200C.
If conventional InGaAlP compound semiconductor light emitting elements can be enhanced in light extraction efficiency, they must be effectively usable to various applications. Heretofore, however, it has been believed very difficult to enhance the light extraction efficiency more than that of the element of FIG. 16. This is because, in the element of FIG. 16, the second side surface 200B and the fourth side surface 200D were believed to be difficult for making depressions and protrusions.
More specifically, in case of the element of FIG. 16, the first side surface 200A of the GaP substrate 200 is (1-11) oriented, but the second side surface 200B is (111) oriented. Thus the first side surface 200A is relatively easy to process for forming depressions and protrusions by wet etching, but the second side surface 200B is difficult to process by the same method. Similarly, the third side surface 200C opposed to the first side surface 200A is (11-1) oriented and relatively easy to process for forming depressions and protrusions, but the fourth side surface 200D opposed to the second side surface is (1-1-1) oriented and difficult to process for forming depressions and protrusions. Even when the slanting angle of the side surfaces is 60°, although the first side surface 200A and the third side surface 200C are relatively easy to process for making depressions and protrusions, the second side surface 200B and the fourth side surface 200D are difficult to process.
A such, in case of forming depressions and protrusions on side surfaces of the substrate 200, since a variety of crystal orientations appear on the respective side surfaces, it is difficult to make uniform depressions and protrusions by chemical etching whose etching rate varies with the surface orientation. Therefore, no structure has been realized in which all of the side surfaces have depressions and protrusions. Accordingly, it has been the common technical knowledge that any more light extraction efficiency than that of the element of FIG. 16 is very difficult to attain.